Mobile communications networks were initially conceived for enabling voice communications, similarly to the wired, Public Switched Telephone Networks (PSTNs), but between mobile users.
Mobile communications networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese counterparts).
The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Messaging System (SMS) and Multimedia Messaging System (MMS) services, and Internet connectivity services have been made available in the last few years.
Mobile communications networks started as Circuit-Switched (CS) networks, similarly to the PSTN; this scheme is satisfactory for supporting voice communications, but offer very poor data exchange capabilities, because the bandwidth that can be allocated for a given user is necessarily limited. In order to exchange relatively large amounts of data, it is preferable to adopt a Packet-Switched (PS) scheme, like in computer networks and, among them, the Internet.
In order to overcome the limitations of conventional, CS mobile communications networks such as the GSM networks, so as to enable mobile users efficiently exchanging, through their mobile phones, relevant amount of data and enjoy the navigation through the Internet, PS network infrastructures have been added to the CS mobile communications networks. One of the solutions that have acquired a significant popularity is the General Packet Radio Service (shortly, GPRS). The GPRS, essentially a GSM add-up, is a digital mobile phone technology compatible with GSM networks (actually, built on the existing GSM network architecture) that supports packet-based data communication, thereby enabling data transfer at a speed higher than that allowed by pure GSM. Mobile phones (or User Equipments—UEs) capable of accessing both the CS and the PS mobile networks have been put on the market, and are currently commercialized.
Initially, PS mobile network infrastructures were conceived to allow mobile users to access a packet-based network, typically the Internet, and exploit services offered by specific Internet servers. As an evolution of this approach, the 3rd Generation Partnership Project (3GPP) has set forth specifications (particularly, the 3GPP Technical Specification TS 23.228, which is to be considered enclosed herewith by reference) for defining an open IP (Internet Protocol) based (i.e., packet-based) service infrastructure (the so-called IP Multimedia Subsystem—IMS) that will enable an easy deployment of new rich multimedia communication services mixing telecom and data services, and particularly person-to-person IP multimedia services, that are expected to be very attractive for customers.
In a scenario in which a generic mobile UE can access, at the same time, both a CS mobile communications network and a PS mobile communications network, particularly an IMS PS network, combining services offered by the CS network (e.g. CS calls) with the IP-based services, made available through the PS network, particularly person-to-person IP multimedia services, may significantly extend the mobile communications capabilities.
For the purposes of the present description, by “combinational service” there is intended a mobile communication service being a combination of services delivered through at least one CS session and at least one concurrent PS session.
For example, combinational services may allow a user speaking with another user, using the CS domain, and, at the same time, using the PS domain, sending to the other user a file, e.g. an image, or playing any sort of game with him/her (gaming).
Up to now, the implementation of combinational services completely relies on the functionality of the UEs.
The general and specific architecture of mobile communications networks (particularly, Public Land Mobile Networks—PLMN) is defined in the 3GPP TS 23.002, which is to be considered enclosed herewith by reference. According to this specification, the accesses of a user to both the PS and CS domain of a mobile communication system are used separately in order to provide independent services to the end user, such as voice calls, for the CS domain, and data service, e.g. Web browsing, for the PS domain. To this end, the CS and PS domains are handled through their own call/session signalings: for the CS domain, they may be of the type specified in the 3GPP TS 24.008 (or, alternatively, signalings complying with the Q.931 or X.31 protocols, or other UNI—User to Network Interface—protocols); for the PS domain, the signaling is compliant to the SIP (Session Initiation Protocol), an IETF (Internet Engineering Task Force) proposed standard for setting up sessions between one or more clients in IP-based networks.
Based on this scheme, in order to set-up a combinational service the generic UE of a mobile communications system including a CS and a PS networks has to simultaneously set up a CS call and a PS session, and needs to be capable of performing a coordination of the CS call and PS session signalings.
More particularly, the UE sets up a CS call and a PS session, that, from the network viewpoint, are completely unrelated, and only the UE is responsible for performing a coordination of the CS and PS signaling. In other words, only the UE is actually aware of the fact that the session in the PS domain is joined to, or combined with, the CS call. On the contrary, the network (and its operator), is completely unaware of this fact, the PS session and the CS call being not linked in any way with each other. Some problems of this implementation of combinational services have been addressed by the Applicant in a submission entitled “Discussion on ‘Combining CS with IMS’”, made for the 3GPP TSG-SA2 Meeting #40, which was held on 17-21 May 2004.
In particular, in that document the Applicant made a distinction between two possible approaches to combinational service implementation, respectively called “CS/CSB” and “IMS/CSB” (acronyms that stand for “Circuit Switched/Combining CS Bearer with IMS” and “IMS/Combining CS Bearer with IMS”).
Essentially, as explained in that document, the CS/CSB approach is characterized by the re-use of existing CS domain service logic, where the set-up of a voice media is done via legacy CS call control, i.e. 3GPP TS 24.008 signaling, as mentioned in the foregoing. Thus, the call & bearer control for voice services is kept on the CS domain (24.008 signaling), the session control for IMS services is done via SIP/IMS signaling, and the bearer control for non-conversational class IMS services is done via PS domain.
In the IMS/CSB approach, instead, IMS services are offered reusing IMS service logic at control plane, and making use of “CS domain bearers” rather than traditional “PS domain bearers” at bearer plane, each time a real-time media component has to be set-up. Thus, the session control for all services (including voice) is done via SIP/IMS, the bearer control for conversational media is done via CS signaling (3GPP TS 24.008), and the bearer control for non-conversational media is done via PS domain.
The Applicant observed in that document that despite the CS/CSB approach is simpler than IMS/CSB, it should only be considered as an (optional) intermediate, easier step towards the latter approach, due in particular to the very limited level of control that the network can apply to the different CS and PS media with respect to the IMS/CSB approach, since voice media and non-voice media are handled separately by different core network domains.
In a submission entitled “Combining CS bearers with IMS”, made by Vodafone UK for the 3GPP TSG-SA2 Meeting WG2 #39, which was held on 19-23 Apr. 2004, after having explained that, currently, 3GPP's IMS specifies the use of SIP signaling to establish sessions using PS bearers to carry the media, there is stated that quality of service and radio efficiency issues, associated with trying to use GPRS bearers for real-time IP sessions, may delay the rollout of IMS for real-time services. In that document Vodafone UK proposes a solution and discloses a potential mechanism for using SIP signaling to set up a session that will use a CS bearer to carry the media. In particular, a SIP Application Server (AS) is used that acts in a back-to-back user agent mode and which can control a media gateway.